Abstract: The invention relates to a method for treatment of a sulphide-containing spent caustic via chemical conversion of the sulphides, wherein the method is comprised of the following steps: introducing the spent caustic into a reaction chamber (18) that is comprised of at least one acoustic power converter (22) and one cavitation element (20), treating the spent caustic with ultrasound from the at least one acoustic power converter (22), feeding a gas mixture containing ozone into the reaction chamber and distributing the gas mixture containing ozone in the spent caustic using the cavitation element (20), wherein the sulphide-containing spent caustic is converted with the gas mixture containing ozone, forming non-sulphide-bearing, inorganic sulphur compounds.